1. Field of the Invention
The present invention relates to an optical polymer composition, and more particularly to a composition for use in optical communications.
2. Description of the Related Art
A wavelength range of light of interest in optical communications has shifted from 800 nm to 1,550 nm, which belongs to the near infrared wavelength range. Thus, it is extremely desirable to manufacture an optical communication device using a material which minimally absorbs light of wavelengths in the near infrared wavelength range.
In general, for an optical lens or an optical substrate such as a compact disk a polymer is used. Research is currently being conducted into use of such a polymer as a material for an optical waveguide for light propagation in the near infrared wavelength range.
However, most polymers absorb light of 1,000-1,700 nm which correspond to the near infrared wavelength range. Such absorption of near infrared wavelength by the polymer is caused by overtone of harmonics due to stretching and deformation vibrations of carbon to hydrogen bonds (C--H) in alkyl, phenyl and other similar functional groups. Thus, using a conventional polymer as a material for an optical waveguide in the near infrared wavelength range results in a large optical loss. In order to reduce optical loss, the light absorption wavelength of the polymer should be shifted from the near infrared light wavelength region to a longer or shorter wavelength region. To this end, the method of substituting hydrogen of the carbon and hydrogen (C--H) bond by fluoride (F) or heavy hydrogen (D) has been suggested.
The method of substituting the hydrogen of C--H bond by D does not yield a material suitable for an optical communications device at the wavelength of 1,500 nm because the material having carbon and heavy hydrogen (C--D) bond absorbs much light of 1,500 nm. On the other hand, the method of substituting the hydrogen by F can minimize optical loss in light absorption at a wavelength of 1,000-1,700 nm.
Another requirement of an optical material for manufacturing an optical device such as an opto-electronic integrated circuit (OEIC), an opto-electrical mixed wiring board(OEMWB), a hybrid integration device, a plastic optical fiber or a multi-chip module (MCM) is thermal stability at 250.degree. C. lasting at least for 30 minutes. Because such thermal resistance of the optical material is very important, glass transition temperature, thermal decomposition temperature, thermal expansion coefficient and birefrigence of the optical material should be carefully considered.
Polyimide has been well known as a polymer having excellent thermal stability. Since polyimide is stable at a high temperature of about 400.degree. C., great efforts to utilize polyimide as a material for optical communications have been consistently made. However, generally, since a conventional polyimide has many C--H bonds in its molecule structure, it exhibits a large optical absorption loss in the near infrared region. To overcome this problem, recently, a method in which hydrogen in C--H bonds of a polyimide is partially or entirely substituted by fluoride has been proposed.
However, if hydrogen is substituted by fluorine, the refractive index of the polymer is decreased. Here, the content of fluoride in the polymer is proportional to the decrease in the value of the refractive index. Thus, since a polyimide obtained by substituting hydrogen in the C--H bonds by fluoride, that is, a fluorinated polyimide, has a low refractive index, in the case of using the same as a core, the range of selection of materials capable of being used for cladding becomes narrow.
Also, the higher the content of fluoride in the polyimide is, the lower the surface tension of a composition containing the polyimide is. Thus, it is difficult to coat such a composition on a substrate and the adhesion of a film comprised of the composition is poor. As a result, film characteristics are deteriorated and the film formed thereby is very fragile.